IPhone 12 Pro won’t stop vibrating

My mum’s iPhone 12 Pro started vibrating randomly and would not stop. It continued to do so throughout a call, when asleep, when accessibility vibrations were turned off and when vibrate on ring & silent were turned off. The only thing that got it to stop was shutting it down. It appears to have stopped after turning it back on, but is this a fault or a setting I need to turn off?

my mum has been very unlucky with her phones. This one is 7 days old, and her previous iPhone 12 Pro was exchanged at 14 days old due to the ringtone suddenly not working. Any advice?